One-pot synthesis of CdS/CeO2 heterojunction nanocomposite with tunable bandgap for the enhanced advanced oxidation process

Herein, a binary nanocomposite CdS/CeO2 has been fabricated via a one-pot co-precipitation method for the degradation of Rose Bengal (RB) dye. The structure, surface morphology, composition, and surface area of the prepared composite were characterized by transmission electron microscopy, scanning electron microscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy, Brunaur–Emmett–Teller analysis UV–Vis diffuse reflectance spectroscopy and photoluminescence spectroscopy. The prepared CdS/CeO2(1:1) nanocomposite has a particle size of 8.9 ± 0.3 nm and a surface area of 51.30 m2/g. All the tests indicated the agglomeration of CdS nanoparticles over the surface of CeO2. The prepared composite showed excellent photocatalytic activity in the presence of hydrogen peroxide under solar irradiation towards the degradation of Rose Bengal. Near to about complete degradation of 190 ppm of RB dye could be achieved within 60 min under optimum conditions. The enhanced photocatalytic activity was attributed to the delayed charge recombination rate and a lower bandgap of the photocatalyst. The degradation process was found to follow pseudo-first-order kinetics with a rate constant of 0.05824 min−1. The prepared sample showed excellent stability and reusability and maintained about 87% of the photocatalytic efficiency till the fifth cycle. A plausible mechanism for the degradation of the dye is also presented based on the scavenger experiments.

While working with conventional iron-based Fenton reagents, the pH must be acidic for optimum efficiency; however, CeO 2 -based photocatalysts can operate at neutral pH, lowering operating costs 23 .
Several researchers have investigated the photocatalytic properties of CdS and CeO 2, but individually, these compounds do not possess adequate photocatalytic efficiency under visible light because of their larger bandgap and rapid charge recombination rate 24 . Some researchers examined binary and ternary nanocomposites of CdS and CeO 2 for the removal of organic pollutants , but they are effective only in aqueous solutions, and they are not very effective in removing highly concentrated dyes from wastewater. The photogenerated charge transfer via an internal electric field often might boost the photocatalytic activity in nanocomposites based on a solid p-n heterojunction interface 25,26 . One of the most efficient ways to encourage charge separation is using CeO 2 and other semiconductors to form nanocomposites. This study has attempted to investigate the one-step synthesis of CdS/CeO 2 heterojunction via a facile co-precipitation method and to investigate its photocatalytic performance for the photodegradation of concentrated dyes present in industrial effluents. The crystal structure and optical properties of the prepared composite were analyzed. The photocatalytic activity of the prepared nanocomposite was investigated for the removal of toxic dye RB from the aqueous stream. The deposition of CdS over the surface of CeO 2 resulted in increased surface area and a greater number of active sites, thus enhancing the photocatalytic efficiency of the composite. The mechanism of degradation of RB over the surface of CdS/CeO 2 under sunlight was investigated. The effect of hydrogen peroxide on the photocatalytic efficiency of the prepared composite is also reported.
Synthesis of CdS/CeO 2 nanocomposite. The CdS/CeO 2 nanocomposite was synthesized via a onepot co-precipitation method. Different amounts of CdCl 2 and Ce(NO 3 ) 3 .6H 2 O were taken in a doubled-necked round bottom flask to maintain a CdS:CeO 2 ratio of 1:1, 1:2, 1:3, 2:1, and 3:1, dissolved in a minimum amount of distilled water and added a few drops of dilute NaOH. The solution was stirred for 1 h at 90 °C. The nitrate ions react with water molecules generating OHions (Eq. 4) which further react with Ce +3 ions resulting in the formation of Ce(OH) 3, which is highly unstable and converted into CeO 2 immediately (Eq. 5). Then added 15% 30 mL of Na 2 S dropwise, an instant source of S 2− ions. Due to a low solubility product of cadmium sulfide, CdS is precipitated (Eq. 6) over the surface of CeO 2 . Furthermore, CdS and CeO 2 can be precipitated easily due to a low solubility product of CeO 2 (K sp = 7 × 10 −21 ) and CdS (K sp = 8 × 10 −27 ) compared to Ce(OH) 3 (K sp = 1.5 × 10 −20 ), Ce 2 S 3 (K sp = 4.4 × 10 −20 ) and Cd(OH) 2 (K sp = 2.5 × 10 −14 ) 27 . After adding sodium sulfide, the reaction was continued at the same temperature for another 2 h and then allowed to cool at room temperature. The final product was centrifugated several times, washed with distilled water and ethanol, and dried at 50 °C for 2 h. A schematic illustration of the synthesis process of CdS/CeO 2 nanocomposite is presented in Fig. 2. Pristine CdS was synthesized by the same method with some slight modifications. Initially, 1 mmol of CdCl 2 was dissolved in a minimum amount of distilled water for 15 min at room temperature and then CdS nanoparticles were precipitated by adding 15% 30 mL Na 2 S solution. The resulting yellow precipitate was stirred for 2 h, washed with distilled water several times, and dried at 50 °C for 2 h. For the synthesis of pristine CeO 2 nanoparticles, 1 mmol of Ce(NO 3 ) 3 ·6H 2 O and 3 mmol of NaOH were dissolved separately in 20 mL of distilled water. The NaOH solution was added dropwise to the cerium salt solution, and the resulting purple solution was stirred at 90 °C for 3 h. The pale white precipitate was collected, washed with distilled water, and dried at 50 °C for 2 h.
Characterization. The crystal phase of the prepared sample was characterized by powder X-ray diffraction using Phillips X'Pert Pro diffractometer with Cu Kα radiations of wavelength 1.54056 Å and a scan speed of 2°/min at room temperature. The microstructure and surface morphology of the sample were analyzed using a high-resolution transmission electron microscope (JEM-2100). The elemental composition of CdS/CeO 2 nanocomposite was analyzed by X-ray photoelectron spectroscopy (XPS) using AXIS ULTRA X-ray photoelectron spectrometer. The specific surface area and the pore structure of the sample were evaluated from nitrogen adsorption and desorption isotherms recorded by Quanta Chrome Nova 1000 gas adsorption analyzer. An FEI QUANTA FEG 200 high-resolution scanning electron microscope was used to record the SEM and EDAX. A Varian Cary eclipse fluorescence spectrophotometer was used to measure the PL intensities. All the UV adsorption studies were performed in Genesys 10S UV-vis spectrophotometer in 1 cm quartz cell from 400 to 800 nm wavelength at a speed of 600 nm-min.
Assessment of photocatalytic activity. The photocatalytic activity of the CdS/CeO 2 nanocomposite has been investigated by monitoring the degradation of Rose Bengal (RB) dye under solar irradiation. All the photocatalytic tests have been performed in 100 mL glass beakers under natural sunlight. The reaction mixture was stirred in the dark for 30 min to attain adsorption-desorption equilibrium, then kept in sunlight for 60 min. The photon lux was calculated using HTC LX-101 A luxmeter and found to be between 84,000-92,000 lx between 11 AM to 1 PM. The average radiation intensity at a reaction mixture's surface was thus found to be 695.2 W/ www.nature.com/scientificreports/ m 2 . The average reaction temperature was 32 °C. The progress of degradation was monitored by withdrawing the suspension solution for 10 min specific time intervals and taking the maximum absorbance of RB (544 nm). The degradation efficiency (%) was calculated by the following equation: where I c (ppm) and I t (ppm) are the initial concentration and concentration at time t of RB. The kinetics of the photodegradation process was evaluated using the following equation: where I c and I t are concentration (ppm) at time t = 0 and t = t and k is the pseudo-first-order rate constant.

Results and discussion
Characterization. X-ray diffraction analysis. The structural properties and phase compositions of the synthesized nanocomposites with different molar ratios of CdS:CeO 2 were analyzed by X-ray diffraction analysis using Philips X'PERT with Cu-Kα radiation having a scan speed of 2°/min at 25 °C. The X-ray diffraction pattern of the synthesized CdS/CeO 2 nanocomposites is represented in Fig. 3a. The peaks occurring at 2θ = 26. As seen in Fig. 3, as the molar ratio of CeO 2 increases, the intensity of the peak at 2θ = 28.6° of CeO 2 increases gradually at the same time, the intensity of the peak at 2θ = 28.6° of CdS decreases. Similarly, for the composites (7) Degradation Efficiency(%) = I C − I t I C × 100 www.nature.com/scientificreports/ with increased CdS ratio, the intensity of the peak at 2θ = 28.6° of CdS also increases, indicating successful fabrication of CdS/CeO 2 nanocomposites. The average crystallite size of CdS/CeO 2 (1:1) nanocomposite was calculated by using Debye-Scherrer's equation, where D is the crystallite size in nm, k is the shape factor (0.89), λ is the wavelength of Cu-Kα radiation (λ = 1.54056 Å), β is full width at half maximum (FWHM) of the particular peak, and θ is the Bragg's angle. The average crystallite size was calculated to be 8.9 nm.
Optical properties. The optical response of CdS, CeO 2 , and CdS/CeO 2 nanocomposite with different molar ratios was examined by UV-Vis diffuse reflectance spectroscopy (UV-DRS), as shown in Fig. 3b. The absorption spectra of CdS showed a broad absorption band ranging from about 260-500 nm with an absorption edge at 600 nm, while pristine CeO 2 showed a narrow spectrum in the UV range with an absorption edge at 450 nm, reflecting a high bandgap of CeO 2 . All the prepared CdS/CeO 2 nanocomposites with different molar ratios showed a similarly broad spectrum in the UV-visible range, from around 260 nm and extending to 500 nm, indicating outstanding light harvesting properties of the prepared photocatalyst. It is worth mentioning that no obvious change in the absorption spectra of CdS/CeO 2 nanocomposites with different molar ratios was observed, indicating that CdS is grown over the surface of CeO 2 and not incorporated within the lattice 28 . Furthermore, a decrease in the absorption edge with increasing CeO 2 content signifies that the light absorption tendency of the nanocomposite is reduced as CeO 2 can only absorb UV light 29 . The bandgap of the prepared nanocomposite was calculated using the Mott equation, i.e. αhυ ∝ (hυ-E g ) 230 , where α is the absorption coefficient, h is the Plank constant, and υ is the wavenumber. Regarding Tauc's plot (inset Fig. 3c), the bandgap of CeO 2 was calculated to be 2.8 eV, while CdS showed a comparatively low bandgap of 2.3 eV, indicating good visible light absorption properties 31 . The bandgap was slightly reduced for the CdS/CeO 2 nanocomposites, as all the samples showed a comparable bandgap from 2.4 for a (1:1) ratio to 2.6 eV for a (1:3) molar ratio. The UV-DRS analysis suggests that the CdS/CeO2 nanocomposite bandgap could be tuned by adjusting the molar ratio of CdS/CeO 2 . The incorporation of CdS with CeO 2 could reduce the bandgap and improve the photocatalytic activity of the nanocomposite in the visible region.
Photoluminescence (PL) spectroscopy is an important technique to access the visible light activity of the prepared nanocomposites. Pristine CeO 2 showed the highest intensity in the PL spectrum suggesting the highest charge recombination rate of charges (Fig. 3d). The intensity of the PL peak decreases with the introduction of CdS with CeO 2 . The decrease in the PL intensity suggests that the charge recombination rate was significantly reduced, which is necessary for higher photocatalytic activity 22 . The CdS/CeO 2 (1:1) nanocomposite showed the lowest intensity in the PL spectrum, indicating that the charge recombination is slowest and was expected to show the highest photocatalytic activity.
Morphological studies. The surface morphology of the synthesized CdS/CeO 2 (1:1) nanocomposite was investigated using SEM. As illustrated in Fig. 4a,b, pure CdS nanoparticles showed spherical morphology, which could also be seen in TEM images; on the other hand, pure CeO 2 showed small irregular cubic particles. The SEM images of CdS/CeO 2 (1:1) nanocomposite showed agglomeration of CdS nanospheres over the CeO 2 surface (Fig. 4c). The TEM micrograph also showed the agglomeration of CdS nanoparticles over the cubic CeO 2 surface. Two types of morphologies could be seen in the TEM images. On closer inspection, the TEM micrograph ( Fig. 4d) revealed the inner part consists of the cubic CeO 2 nanoparticles, and the outer greyish part is the CdS nanoparticles 32 . The high-resolution TEM (Fig. 4e) showed two types of interplanar lattice spacing on the surface, 0.33 nm, and 0.29 nm, which corresponds to the (111) and (200) planes of CdS, while the inner part showed consistent lattice fringes of 0.31 nm corresponding to (111) plane of CeO 2 . Furthermore, the formation of heterojunction between CdS and CeO 2 can be clearly seen in Fig. 5c. Thus, TEM studies confirm the formation of CdS/CeO 2 structure where CdS nanoparticles were anchored over the surface of CeO 2 . Due to the addition of NaOH, first CeO 2 nanoparticles were formed, and after the addition of Na 2 S solution, CdS nanoparticles were precipitated over the surface of CeO 2 . The bright and concentric rings of the SAED pattern revealed the high crystallinity and polycrystalline nature of the synthesized CdS/CeO 2 (1:1) nanocomposite material, and the lattice planes (220) of CeO 2 and (111) and (200) planes of CdS were identified and marked (Fig. 4f).
XPS analysis. The elemental composition and oxidation states of metals were determined by XPS analysis of the prepared nanocomposite. The XPS survey spectrum (Fig. 5a) demonstrated peaks corresponding to Ce, Cd, O, S and C, and no impurity was found, confirming the samples' composition and high purity. The highresolution XPS survey spectrum of Cd 3d and S 2p (Fig. 5b,c) showed two broad peaks at binding energies 411.6 eV and 405.1 eV for Cd and 168.2 eV and 161.5 eV for S, which can be indexed to Cd 3d 3/2 , Cd 3d 5/2 , S 2p 1/2, and S 2p 3/2 respectively belonging to Cd +2 and S −2 oxidation states in CdS. The oxidation state of cerium oxide is an important parameter in determining its structure. The tetravalent Ce +4 in CeO 2 is in a cubic fluorite structure 33 . The Ce 3d level consisted of several peaks centered at 916.4 eV, 906.8 eV, 900.6 eV, 898.2 eV, 885.2 eV, and 882.2 eV, respectively (Fig. 5d). It was reported that peaks at 882.2 eV, 898.2 eV, 906.8 eV, and 916.4 eV are related to the Ce +4 oxidation state, and the peaks at 885.2 eV and 900.6 eV are characteristics of Ce +3 oxidation state 34 . According to Fabris et al. 35 , partially reduced mixed-phase ceria is an intermediate phase. The presence of Ce +3 could be due to the presence of oxygen defects or a small amount of Ce 2 O 3 in the sample 28 . This indicates   www.nature.com/scientificreports/ oxygen can be captured by Ce +4 ions and get reduced to Ce +3 , which confirms the existence of Ce +3 ions in the Ce 3d spectrum 34 . Additionally, energy dispersive X-ray analysis (EDAX) analysis (Fig. 5f) (Fig. 6a-f) also revealed the presence of Cd, Ce, S, and O homogenously dispersed within the nanocomposite. Therefore, XPS, EDAX and mapping analysis confirms the successful fabrication of the CdS/CeO 2 (1:1) nanocomposite. The heterojunction between CeO 2 and CdS results in the generation of highly reactive oxygen species, which improves the photocatalytic activity of the CdS/CeO 2 nanohybrid.
Surface area analysis. The photocatalyst's surface area is a significant factor affecting photocatalytic activity. Generally, a larger surface area is desirable for an ideal photocatalyst. The Brunauer-Emmett-Teller (BET) analysis of CdS/CeO 2 (1:1) was carried out, and the specific surface area of the prepared composite was obtained as 51.30 m 2 /g with pore volume and pore diameter of 0.889 cm 3 /g and 693.00 Å, respectively, which could be due to www.nature.com/scientificreports/ agglomerated CdS nanoparticles 38 . Very high pore volume suggests that the prepared CdS/CeO 2 (1:1) nanocomposite is macroporous in nature. The results concluded that the photocatalytic activity is significantly affected by the surface area and pore size of the nanocomposite 1 . Moreover, the nitrogen adsorption/desorption isotherm of the prepared composite showed hysteresis loops (Fig. 6g). The adsorption isotherm belongs to type IV isotherm, with the hysteresis loop having characteristics of H3 type for the prepared composite.
Photocatalytic activity. Photocatalytic activity of different photocatalysts. The photocatalytic activity of different catalysts (2 mg) towards the degradation of 50 mL 150 ppm RB dye is shown in Fig. 7a,b. Although such high concentrations of dyes are usually not found in wastewater, higher dye concentrations are employed in photodegradation studies for better evaluation of the performance of the photocatalyst 39 . Before starting photodegradation, the dye solution with catalyst was stirred in the dark for 30 min to attain adsorption/desorption equilibrium. A blank experiment was done without a catalyst showing a negligible rate to rule out the possibility of self-photolysis of the dye. The characteristic UV-visible absorption peak of the dye decreased with time in the presence of the photocatalyst, which shows that the dye was degraded under solar irradiation by the photocatalyst.
The photodegradation efficiency of RB dye reached 46.92% for CdS/CeO 2 (1:1) nanocomposite whereas pristine CeO 2 and CdS showed 4.55% and 14.75% degradation within 60 min. However, hydrogen peroxide is a well-known oxidizing agent commonly used in the photodegradation of organic compounds to enhance the photodegradation efficiency. Therefore, a small amount of hydrogen peroxide was added to boost the photodegradation and the amount of hydrogen peroxide was optimized further. www.nature.com/scientificreports/ The photocatalytic activity of different catalysts towards the degradation of 50 mL 150 ppm RB dye in presence of 0.2 mL H 2 O 2 is shown in Fig. 7c,d. The degradation efficiency of RB increased to 12.45% and 24.69% for pristine CeO 2 and CdS, respectively. The results showed that pristine samples do not possess sufficient photocatalytic activity toward the degradation of RB dye. The reason could be due to a wide bandgap of CeO 2, which only absorbs UV light, while the bandgap of CdS corresponds to the visible region, but rapid recombination of charges limits its photocatalytic activity 17 . However, the photodegradation of RB dye was significantly improved, and a maximum degradation of 88.60% could be achieved within 60 min using CdS/CeO 2 (1:1) nanocomposite. The increased efficiency was attributed to the formation of heterojunction between CdS and CeO 2 nanoparticles, which helps to delocalize the photoinduced electrons and holes, as also evident from the PL studies. Furthermore, the photogenerated electrons could react with adsorbed oxygen to produce superoxide radicals, and the holes could oxidize hydroxide ions to hydroxyl radicals in the CdS/CeO 2 (1:1) nanocomposite. We also observed that the photodegradation efficiency was affected by the ratio of CdS and CeO 2 . The best performance was shown by CdS/ CeO2(1:1). This is because pure CeO 2 could not produce sufficient reactive oxygen species. At the same time, a higher amount of CeO 2 suppresses the light-absorbing tendency of CdS, reducing the number of photogenerated electrons to recombine with holes, thereby suppressing the charge transfer 40 . Therefore, CdS/ CeO 2 (1:1) nanocomposite was chosen for further photocatalytic experiments. Table 1 illustrates the degradation performance of different photocatalysts.
Effect of hydrogen peroxide. Hydrogen peroxide (H 2 O 2 ) was reported to enhance the photocatalytic degradation of dyes in wastewater 41 . Therefore, the effect of the amount of hydrogen peroxide on the photocatalytic degradation efficiency of CdS/CeO 2 (1:1) nanocomposite was investigated by varying hydrogen peroxide amounts in the range of 0.2 mL/50 mL to 1 mL/50 mL of dye solution under sunlight irradiation. It is worth mentioning that no significant degradation (< 8%) occurred in presence of a catalyst and H 2 O 2 under the dark suggesting the non-existence of the Fenton reaction in dark conditions. The photocatalytic activity of CdS/CeO 2 (1:1) was improved in presence of H 2 O 2 . The degradation efficiency increased with the amount of hydrogen peroxide to 0.6 mL, but a slight decrease in efficiency was observed beyond 0.6 mL. The degradation efficiency reached 93.65% within 60 min of irradiation in the presence of H 2 O 2 (Fig. 8a), whereas only 46.92% of the dye could be degraded without peroxide in 60 min. The kinetics of photodegradation of RB in the presence of 0.6 mL hydro- www.nature.com/scientificreports/ gen peroxide was examined (Fig. 8b). The effect of hydrogen peroxide on the degradation of RB and the reaction kinetics is illustrated in Table 2. The degradation process follows pseudo-first-order kinetics, and the rate constant was calculated to be 0.047 min −1 . Furthermore, no significant decrease in RB concentration was observed with H 2 O 2 without the photocatalyst ruling out the possibility of self-degradation of RB. The increased photodegradation efficiency in the presence of H 2 O 2 could be attributed to the production of a large number of hydroxyl radicals. The efficiency decreased beyond 0.6 mL because a higher peroxide concentration leads to the capture of hydroxyl radicals by H 2 O 2 to form water and HO 2 · radicals according to the following reactions 42,43 : The reaction of H 2 O 2 with free electrons and O 2 ·− yields additional hydroxyl radicals, which promote the degradation efficiency by increasing the amount of hydrogen peroxide according to Eqs. (10 and 11). Further increment in the amount of hydrogen peroxide causes a decrease in degradation efficiency because excess H 2 O 2 traps hydroxyl radicals, generating HO 2 · which does not play a major role in dye degradation. The trapping of hydroxyl radicals occurs according to Eqs. (12 and 13). A decrease in hydroxyl radicals due to increasing hydrogen peroxide dosage restricts the degradation of RB. Therefore, 0.6 mL H 2 O 2 /50 mL is fixed for further photocatalytic experiments.
Effect of pH. The pH of the solution is a significant factor affecting the photodegradation of dye in the aqueous phase 44 . The effect of pH on the degradation of RB was investigated by keeping the other parameters constant (Initial dye concentration = 150 ppm, Volume = 50 mL, Catalyst dosage = 2 mg and H 2 O 2 dosage = 0.6 mL) and varying pH from 4 to 10. As illustrated in Fig. 9, the degradation of RB increases with an increase in pH, and about 95.12% degradation could be achieved at pH 8 within 60 min of solar irradiation. This could be explained by the fact that under alkaline conditions, more hydroxyl ions are present in the solution, which interact with positively charged holes generating more hydroxyl ions according to the following equation 45 : www.nature.com/scientificreports/ However, at higher pH, more hydroxyl radicals are adsorbed on the photocatalyst surface; therefore, the surface of the photocatalyst becomes negatively charged, thereby inhibiting the generation of hydroxyl free radicals and causing repulsion between the photocatalyst and the dye molecules resulting in a decreased degradation efficiency 46 .
Effect of catalyst dosage. The amount of photocatalyst dosage greatly influences the degradation of pollutants through AOP. To investigate the effect of catalyst concentration, other parameters like initial dye concentration, pH, and H 2 O 2 dosage were fixed at 150 ppm, 8, and 0.6 mL, respectively, and the photocatalyst dosage was varied from 1 mg/50 mL to 5 mg/50 mL. The 2 mg/50 mL catalyst dosage showed 98.6% degradation of the dye solution within 60 min of irradiation with a pseudo-first-order rate constant of 0.05174 min −1 (Table 3; Fig. 10b). However, the degradation efficiency declined after that (Fig. 10a). The initial increase in the degradation percentage could be attributed to an increased number of active sites for the generation of ROS. However, at a higher catalyst dosage (> 2 mg) due to, the collision of nanoparticles in the solution phase blocks the sunlight from penetrating deep into the catalyst surface, decreasing the degradation of RB dye 47 . Hence the photocatalyst dosage was fixed at 0.04 g/L.
Effect of initial dye concentration. To further investigate the effect of the concentration of RB on the performance of the photocatalyst, the initial dye concentration was varied in the range of 5-200 ppm, keeping the other reaction parameters constant. No significant effect on the degradation performance of the photocatalyst was observed up to 190 ppm (Fig. 11a). After that, the degradation efficiency slumped to 96.2% for 200 ppm, which could be attributed to the hindrance in the path of photons due to higher dye concentration 48 . Furthermore, higher dye concentrations will require a greater photocatalyst loading which will further increase the opacity of the solution 49 . Studies claimed that increasing the dye concentration blocks the photocatalyst's surface active sites, inhibiting ROS generation, and thereby decreasing the degradation efficiency 50 . The maximum degradation efficiency of 97.14% for 190 ppm RB could be attained with a pseudo-first-order rate constant of 0.05824 min −1 within 60 min (Table 4; Fig. 11b). Therefore, 190 ppm RB dye was chosen as the optimum dye concentration for further photodegradation tests.

Effect of contact time and synergistic index.
Under optimum conditions of initial dye concentration of 190 ppm, 0.04 g/L photocatalyst dosage, 0.6 mL of H 2 O 2 dosage, and at pH 8, no further increase in the degradation percentage was observed after 60 min of solar irradiation. This could be due to the exhaustion of the surface active (14) HO − + h + → HO · www.nature.com/scientificreports/ sites of the photocatalyst. The highest degradation of 97.14% is reached under optimum conditions (Fig. 12b). Moreover, the degradation efficiency increases up to around 67.85% in the absence of H 2 O 2 in 120 min, and no significant change was observed after that (Fig. 12a).
In order to study the synergistic effect between CdS and CeO 2 , the synergy index under optimum conditions in the presence and absence of H 2 O 2 was calculated using the formula as follows 51 : where, R CdS/CeO 2 , R CdS and R CeO 2 are the reaction rate constants of CdS/CeO 2 (1:1), CdS and CeO 2 , in absence of H 2 O 2 , respectively. Figure 12c,d illustrates the degradation performance of CdS, CeO 2, and CdS/CeO 2 (1:1) nanocomposite in the presence and absence of optimized amount of H 2 O 2 . As illustrated in Table 5, the reaction rate constants of R CdS/CeO 2 , R CdS and R CeO 2 are 0.01262 min −1 , 0.00767 min −1 , and 0.00439 min −1 , respectively, while the rate constants of the same in the presence of H 2 O 2 are 0.05824 min −1 , 0.0134 min −1 , and 0.00630 min −1 , respectively. The synergy index was calculated to be 1.0 in the absence of H 2 O 2 and ~ 3.0 in the presence of H 2 O 2 , indicating that the combination of CdS and CeO 2 and the formation of heterojunction is beneficial for H 2 O 2 activation and the degradation of RB dye.
Scavengers test. The photodegradation reactions are controlled by the concentrations of ROS and the photogenerated charge carriers (electrons and holes) which generate them. Therefore, a few sacrificial agents were added to the reaction cell to trap the ROS and the charge carriers to investigate the effect of these radicals on the degradation of RB dye. Benzoic acid and ascorbic acid are used to trap OH · and · O 2 − radicals. Potassium persulphate (K 2 S 2 O 8 ) and Na 2 EDTA were employed at electron (e − ) and hole (h + ) scavengers. With the addition of 1 mmol of ascorbic acid and Na 2 EDTA, the degradation of RB slumped to 14.7% and 23.2%, respectively (Fig. 13). However, K 2 S 2 O 8 and benzoic acid does not significantly affect the degradation process, indicating that the degradation of RB is mainly due to the presence of · O 2 − and h +52 .  www.nature.com/scientificreports/ Mechanism of the degradation of RB. Based on the above discussions, a schematic mechanism of photodegradation of RB over the CdS/CeO 2 (1:1) surface is shown in Fig. 14. When the fabricated photocatalyst is irradiated under sunlight, the electrons get excited from the valance band (VB) to the conduction band (CB) of CdS forming positively charged h + in the VB due to its lower bandgap. The generated e − immediately gets transferred to the CB of CeO 2 and initiates a redox reaction forming a Ce 4+ /Ce 3+ redox couple which in turn produces ROS contributing to the degradation of RB. Then these electrons interact with adsorbed oxygen generating · O 2 − radicals. The h + in the VB of CeO 2 moves to the VB of CdS, where they react with hydroxide ions generating OH · radicals 53 . Finally, the RB dye molecules adsorbed on the surface of CdS/CeO 2 (1:1) get attacked by these ROS, resulting in its degradation. Furthermore, h + in the VB also reacts with adsorbed RB initiating its degradation process. Thus coupling a low bandgap photocatalyst such as CdS with CeO 2 proved to be beneficial for effective charge delocalization and improved photocatalytic activity. The steps in the photodegradation of organic pollutants by nanocomposite photocatalyst (NP) are illustrated below:    Reusability studies. Reusability is an important factor in evaluating the stability and practical applications of the prepared sample. The as-prepared CdS/CeO 2 (1:1) nanocomposite was investigated for reusability by subjecting it to four consecutive experimental cycles under the same conditions. After each cycle, the sample was filtered, washed with ethanol and distilled water, and dried at 50 °C for 3 h before reusing for the next cycle. As illustrated in Fig. 15, the prepared photocatalyst shows a slight decrease in photocatalytic activity until the fourth cycle. The efficiency was almost the same in the next experiment. In the reusability experiment, the photocatalyst retained about 87% efficiency till the fifth cycle suggesting good stability and reusability of the as-prepared CdS/ CeO 2 (1:1) nanocomposite. The reduction in the degradation efficiency might be due to the blockage of active sites by the degradation products during degradation 54 . Furthermore, the reused catalyst after the fifth cycle was characterized by XRD, EDAX, TEM, and SEM to evaluate its structural and chemical stability. As seen in Fig. 15c, the XRD spectrum showed all the peaks corresponding to CdS and CeO 2 in the reused sample. The XRD suggests that the crystal structure of the reused catalyst was perfectly maintained after the fifth cycle. Additionally, the EDAX (Fig. 15d) spectrum confirms the presence of Cd, Ce, S, and O in the reused catalyst, suggesting that the CdS was not photo corroded during the photocatalytic tests. The formation of heterojunction nanocomposite effectively enhanced the charge separation which suppressed the photo corrosion of CdS 55 . Moreover, the ratio of Cd:S and Ce:O was maintained perfectly, suggesting excellent stability of the prepared photocatalyst. The TEM and SEM images (Fig. 15a,b) also revealed the presence of CeO 2 and CdS in the reused nanocomposite, indicating that the prepared CdS/CeO 2 (1:1) nanocomposite maintained its structure after the fifth run.

Conclusion
The one-step synthesis of CdS/CeO 2 (1:1) nanocomposite was carried out via the co-precipitation method, and the photocatalytic efficiency in the presence and absence of hydrogen peroxide was investigated. Various characterization techniques were used to assign the nanocomposite's structure, composition, and morphology. SEM, TEM, and HRTEM images revealed the accumulation of CdS nanoparticles over a cubic CeO 2 surface. The nanocomposite photocatalyst showed excellent degradation efficiency of 97.14% within 60 min of solar irradiation in the presence of hydrogen peroxide with a pseudo-first-order rate constant of 0.05824 min −1 . This is due to the formation of superoxide radicals and holes during the photocatalytic process. The prepared nanocomposite was